The present invention relates to a method of reducing intensity pulsation in distributed feedback (DFB) lasers, e.g. in DFB fibre lasers.
The utilisation of optical fibre networks in telecommunications is becoming more and more prevalent due to their high bandwidth capabilities. Further, with the recent introduction of erbium doped fibre amplifiers (EDFA) wavelength division multiplexing (WDM) systems are being introduced so as to multiplex multiple channels. The increase in WDM density places more stringent requirements on the principles of operation. This requires laser transmitters with accurate wavelength selection and high wavelength stability, in addition to low power output fluctuations.
Fibre lasers such as Er-doped DFB fibre lasers in general are ideally suitable as they are fully fibre-compatible allowing for very low coupling losses. The potential of DFB fibre lasers as low noise, narrow linewidth sources for WDM systems has been demonstrated recently in digital transmission tests. Further, with a passive temperature-compensated package, the wavelength stability of DFB fibre lasers could be set better than 1 GHz within xe2x88x9220/+80xc2x0 C. temperature range.
However, due to self-pulsation in Er-doped DEB lasers, there exist power fluctuations in the output of such lasers. The origin of self-pulsations is related to ion clustering at high erbium concentrations [Sanchez et. al. Phys. Rev. A, 48(3), 2220-22293]. The clusters act as saturable absorbers with switching time much shorter than the population inversion recovery time and can eventually result in spiking behaviour of the laser.
Reducing the erbium concentration whilst still providing enough gain in a short cavity DFB fibre laser can be achieved by Yb co-doping [Kringlebotn et. al. IEEE Photon. Technology Letters 5(10) 1162-1164 (1993)] which increases the pumping efficiency. However, where it is desired to arrange several DFB fibre lasers in series, this method can have the disadvantage that the Yb dopant absorbs a significant portion of the pumping energy, and therefore separate pumping sources would typically be required.
Stabilisation of the laser against self-pulsations can also be accomplished by resonant pumping [Loh et al, Optics Letters 21(18), 1475-1477 (1996)] or co-pumping [Loh et. al. Optics Letters, 22(15), 1174-1176 (1997)] directly into the metastable Er-ion state, damping down the oscillations of the population inversion. However, this approach has the disadvantage that the pumping wavelength would lie close to the signal wavelength. Presently, sources for wavelengths close to commonly used signal wavelengths of around 1480 nm are quite expensive.
The present invention provides a method of reducing fluctuations in the output power of a distributed feedback laser arrangement incorporating a waveguide structure having a distributed feedback region, a signal amplification region for amplifying an output of the distributed feedback region and a saturable absorption region, the method comprising using light from the distributed feedback region to induce a saturable absorption grating in the saturable absorption region.
The method may be effected in a laser arrangement wherein the saturable absorption region is provided at one end of said signal amplification region.
The method may be effected in a laser arrangement wherein said saturable absorption region forms part of said signal amplification portion.
The method may be effected in a laser arrangement wherein said signal amplification region is in a feedback loop with said distributed feedback region.
Said feedback loop may be formed by coupling a portion of an output of said signal amplification region to said distributed feedback region.
Said distributed feedback laser region may be formed from Erbium doped fibre.
Said signal amplification region may be formed from Erbium doped fibre.
Said saturable absorption region may be formed from Erbium doped fibre.
The feedback loop may provide a phase-conjugated feedback signal to the output of the distributed feedback region.
The feedback signal may provide resonant pumping as well as saturate gain in the distributed-feedback region to the threshold value.
The method may be effected in a laser arrangement wherein a number of said distributed feedback regions are connected in series.
One signal amplification region and one saturable absorption region and one feedback loop may be shared between said distributed feedback regions to form the arrangement.
The method may be effected in a laser arrangement wherein the distributed feedback region comprises a Bragg grating structure.
The Bragg grating structure may comprise a chirped Bragg grating.
The Bragg grating structure may comprise a sampled Bragg grating.
The Bragg grating structure may comprise a phase shifted Bragg grating.
The grating structure may comprise an apodised grating.
The method may be effected in a laser arrangement wherein the waveguide structure comprises a planar waveguide.
The distributed feedback region may be in the form of a planar waveguide.
The signal amplifying region may be in the form of a planar waveguide.
The saturable absorption region may be in the form or a planar waveguide.